The Golden State: Where & How to Live, Secure, Visit, Enjoy and Thrive in California
Practical Electricity And House Wiring
How Fuses Operate.—A fuse is nothing more or less than a short piece of wire or ribbon made of a metal that melts easily, and of a diameter or size which experiment has shown will melt when a specified amperage flows through it, all enclosed in a convenient housing. When the metal melts (when the fuse blows), it interrupts the circuit, exactly as if a switch has been opened or a wire cut. The metal is enclosed in the body of the fuse which is sturdily built so that when it melts it cannot spatter and do damage —the molten metal must stay within the body of the fuse.
Rating of Fuses.—Fuses are rated by the number of amperes they will carry without blowing. The most ordinary size of fuse is rated at 15 amp., protecting No. 14 wire.
This means that such a fuse will carry 15 amp. Forever without blowing. If 16 amp. flow, the wire inside the fuse will get a little hotter, but it will be many minutes before the fuse will blow. If 20 amp. flow, the wire gets hotter more quickly and the fuse will blow after a few minutes. If there is a short circuit and several hundred amperes flow, the fuse will blow in a small fraction of a second.
There are several types of fuses, the most common of which is the ordinary plug fuse of Fig. 76. Standard ratings for plug fuses are 6, 10, 15, 20, 25, and 30 amp., but never more than 30 amp. Cartridge style fuses are shown in Fig. 77 and are available in all amperages; in homes they are seldom used in amperages under 30. All those that are rated at 30 amp. or less are of one physical size, and therefore interchangeable. Those rated at 31 to 60 amp. are all of the same size, but larger than those rated at 30 amp. or less. Those rated at 61 amp. or more are similar but larger, and have knife-blade terminals as shown in the lower half of- Fig. 77.
Time-Lag Fuses.—A piece of wire is damaged when too much current flows through it, as we have already learned. No. 14 wire, for example, can safely carry 15 amp. continuously, and in doing so will ordinarily reach a temperature of about 140° F., which is a temperature which will neither damage the insulation, nor start a fire. If it carries 20 amp. continuously, a much higher temperature will be reached, probably shortening the life of the insulation. If it carries 30 amp. continuously, it will probably become dangerously hot. However, if it carries 35 amp. for a few seconds, and after that carries only 10 amp. or even 15 amp., its temperature will not be seriously affected, nor will there be danger of fire.
